Recently, copper has been introduced in ULSI metallization schemes as a replacement for aluminum due to its lower resistivity and superior electromigration resistance. When copper is used as a conductive path, electrolytic deposition (plating) has emerged as the deposition method of choice for damascene copper interconnects. Electrolytic deposition of a metal, e.g., copper in the case of copper interconnects, is performed by bringing a surface, e.g., one side of a wafer, in contact with a solution containing the ions of the metal to be deposited and supplying electrons to these ions to deposit metal atoms on the surface by a reduction reaction. When the electrons for ion reduction are supplied from a reducing agent present in the solution, this electrolytic deposition process is referred to as electroless deposition. If, in the deposition process, the substrate is electrically connected to an external power supply to deliver electrons, this is referred to as electrodeposition. The substrate, electrically connected in such a way that the metal ions are reduced to metal atoms, is referred to as the cathode. Another electrically active surface, known as the anode, is also present in the electrolyte to complete the electrical circuit. At the anode, an oxidation reaction occurs that balances the current flow at the cathode, thus maintaining electrical neutrality in the solution. In the case of copper plating, cupric ions removed from the electrolyte at the wafer cathode are replaced by dissolution from a solid anode containing copper. The action of depositing the metal can be combined with the action of mechanical polishing in order to avoid the accumulation of deposited material.
In IC technology, the challenge arises in depositing copper into very high-aspect-ratio sub-micron openings such as trenches, vias, and holes without forming voids in the plated metal and wherein the plated metal possesses the appropriate materials properties with respect to, for example, grain size, impurity content, stress, electrical resistivity, roughness, hardness, and ductility. The ability to achieve this desired defect-free filling (also known as “superfilling” or “bottom-up fill”) of sub-micron damascene structures by plating is largely dependent on the influence of additives, organic or inorganic in nature, or a combination, which are added to the plating bath containing the ions of the metal to be deposited (e.g., copper ions). State of the art commercial additive systems (e.g., additives for interconnect applications) generally include a combination of proprietary additives containing sulfur, nitrogen and/or oxygen functional groups. The additives can include brighteners, levelers, or carriers, as are known in the art. It is generally noted that the addition of halide ion, typically chloride or bromide, is preferred, if not necessary, to obtain good deposits.
A “bright deposit” is a deposit which has a highly reflective surface gloss, and brighteners are additives which, when added to a copper plating solution, improve the brightness of the deposit. Brightening is usually defined as the ability of a plating solution to produce fine deposits which consist of crystallites smaller than the wavelengths of visible light and having oriented grain structure. Additives acceptable for use as brighteners are well known in the art.
The term “leveled deposit” describes a deposit whose surface is smoother than that of its substrate. Thus, “leveling” denotes the ability of a plating bath to produce deposits relatively thicker in recesses and relatively thinner on protrusions, thereby decreasing the depth of surface irregularities (planarization). In the art of damascene interconnect technology, the filling/leveling of very high-aspect-ratio sub-micron features is usually referred to as “superfilling,” “super conformal plating” or “bottom-up fill.”
Brighteners and levelers may include sulfur and/or nitrogen containing molecules. Typically, sulfur containing compounds may include sulfonated or phosphonated organic sulfides such as, for example, 4,5-dithiaoctane-1,8-disulfonic acid, 3-mercaptopropylsulfonic acid, or their salts. These compounds give rise to a depolarization of metal ion discharge, such as copper ion discharge, hence, they may also be referred to as “depolarizers”. A brightener or leveler can have a depolarizing effect. Another typical example of a depolarizer is thiourea or its derivatives, which, depending on its concentration, may have a polarizing or depolarizing effect on copper ion discharge. Nitrogen-containing molecules which may have a depolarizing effect include, for example, phthalocyanine compounds (e.g. Alcian Blue), or phenazine azo dyes (e.g. Janus Green B). Some such additives were found to act as brightener and leveler simultaneously.
Brighteners/levelers are usually used in combination with carriers (also referred to as “suppressors”). Suppressors are typically polymers containing polyether components, such as polyethylene glycol, polypropylene glycol, their block copolymers, polyether surfactants, or alkoxylated aromatic alcohols. Also, halide ions, such as chloride or bromide ion, are typically added to the plating bath. Carriers typically act by suppressing the electrolytic copper deposition, especially in combination with chloride or bromide ions, which affect the adsorption behavior of carriers and brighteners/levelers.
The synergistic effects between the additives results in local change and balance of acceleration and suppression of copper deposition and plays a central role in achieving the desired fill profiles for very high-aspect-ratio features. The additives also influence the materials properties of the deposit.
In state-of-the art plating, the depletion of the additives in the copper plating bath over time drastically complicates the manufacturability of the copper plating process. Especially problematic is the decomposition of the brightening/leveling compounds in a typical commercial electrolytic plating bath. These compounds are typically prone to decomposition at a high rate in solutions containing copper ions in contact with metallic copper. Thus, even when the electroplating solution is not in use but is left in contact with the copper-containing anode, the decomposition of the additive continues. Therefore, an appropriate electrolyte management system with precise and fast feedback control of the constituents of the plating solution is required to maintain the desired filling and materials properties of the deposit. Such a management system necessitates the application of on-line and real-time analysis of the plating electrolyte, e.g., by cyclic voltammetric stripping, and continuous feedback control of the electrolyte composition by dosing and spiking of additives. The concentration of the additives can be controlled this way. However, a continuous accumulation of decomposition products and impurities still occurs in the plating bath, affecting the filling and materials properties of the deposit.
Another problem encountered in state-of-the-art plating is the formation of bumps of deposited material on top of recessed areas. This is believed to be due to the accumulation of compounds which have a depolarizing effect on the metal ion discharge (depolarizers).
Several methods may be used to overcome or at least reduce the extent of this problem. In one method, after filling of the recess, a deplating step is performed to redistribute the adsorbed species over the surface. After deplating, the remaining portion of the layer is deposited. Another method is to perform a thermal anneal after filling the recess to desorb the additives from the surface. After this thermal anneal, the remaining portion of the material is deposited. Another method to reduce bump formation is to apply mechanical polishing simultaneously with the action of deposition.
U.S. Pat. No. 6,319,831 describes a method for depositing copper in high aspect ratio contact/via openings in integrated circuits. The copper is deposited from a copper plating electrolyte containing brighteners and levelers. The first copper layer is plated at low current density. Since the concentration of the brightener decreases in the base regions of the openings, the brightener is replenished in these regions while the current is stopped. In a subsequent step, a high current is applied during copper deposition.